A two-dimensional model of interior flow was constructed for underwater missile concentric canister launcher (CCL). Two-phase flow problem was reduced to a simple high-speed gas jet. The numerical simulation indicates that the results such as the missile's velocity and displacement calculated with single-phase high-speed gas jet model and VOF model are not very different. The single phase high-speed gas jet model can give the flow details in the interior flow for underwater launched missile well and tru-ly. The simulation based on VOF model costs much more time than that based on single-phase high-speed gas jet model. A simple model of the interior flow for underwater launching is feasible.